What if the divorce decree did not state who can claim the kid on taxes and i pay child support can i claim?

It is who ever has custody of the child. There is really nothing you can do unless you are the one with the custody or the both of you work out some kind of arraignment.

Whether or not she works has nothing to do with who can claim the kids. They live with her, which means SHE is their primary caregiver. SHE is the one who is responsible for ensuring they are cared for. SHE claims them, as they are her dependents. You contribute a minute fraction of what it costs to support them. When you can prove you are contributing more than she is, you can claim them. (And again, the fac that she doesn't work doesn't mean she isn't contributing more than you are. $300 doesn't begin to cover the cost of shelter, food, clothing, etc.)

You have to work something out. Legally though it's either you or her to claim the child. No one else can do so.

it has to be her unless she is living with her parents. You can't claim them if you don't have them! They have to be living with you for over 6 months for you to be able to claim them as dependents